This invention relates generally to a method for making x-ray masks and the like.
X-ray masks of the type used in fabricating bubble memories must be capable of delineating sub-micron structure in x-ray sensitive resist films in order to produce satisfactory results in x-ray lithography. The thickness of the mask membrane is critical to achieving good results and it is desirable that the mask membrane be as thin as possible. The thickness of the membrane is a compromise between absorption capabilities and mechanical strength. For example, in aluminum radiation, membranes of less than six microns in thickness are utilized. This has led to the development of special techniques for forming and patterning thin, strong mask membranes.
Typically, the pattern to be copied is formed in gold, which is a good absorber of aluminum radiation, and x-ray masks have been fabricated on silicon, Mylar, Kapton and polyimide membranes. Polyimide membranes are desirable because they combine the surface qualities of silicon with the strength of Mylar films and have high transmission quality.
A method for the production of thin polyimide membrane x-ray lithography masks have been disclosed by D. C. Flanders and Henry I. Smith in a paper to be published in the Journal of Vacuum Science and Technology as part of proceedings of the Fourteenth Symposium on Electron, Ion and Photon Beam Technology. According to this process, thin membranes of polyimide are formed by spinning polyamic acid on a glass substrate and polymerizing in situ. The glass substrate acts as a holder and heat sink during formation of gold absorber patterns on the polyimide. A support ring is then bonded to the polyimide and the glass is etched away in dilute hydrofluoric acid. There is thus obtained polyimide membranes with thicknesses of from 0.5-5 microns.
One shortcoming of the method described above is that it involves the transferring and bonding of the membrane to a support frame. These steps subject the very thin membrane to the possibility of mechanical distortion which can impair the accuracy of the mask in use.